A voltage converter can be used to supply power to various kinds of electrical devices, and operates by converting an input voltage received at its input terminal to an output voltage provided at an output terminal of the voltage converter. A voltage converter can take one of many different forms, which may be selected depending on the requirements of the application at hand. For example, the switching voltage converter (also known as a switched mode power supply, SMPS) is a well-known type of voltage converter that is well-suited to use in personal computers and portable electronic devices such as cell phones, for example, owing to its small size and weight, and high efficiency. A switching voltage converter achieves these advantages by switching one or more switching elements such as power MOSFETs at a high frequency (usually tens to hundreds of kHz) to convert the input voltage to an output voltage. A voltage converter may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC), for example.
There are, however, applications whose requirements cannot be met by a single voltage converter. For example, the demand for ever faster and more complex signal processing has fuelled the need for new generations of signal processing systems having multiple high-performance processors, which are characterised by their need for multiple low supply voltages, high current demand and tight supply voltage regulation requirements. These needs are met by power supply systems such as the Intermediate Bus Architecture (IBA) power supply system, which employ a multi-stage voltage conversion arrangement having multiple voltage converters to derive a number of tightly-regulated voltages from an input power source.
FIG. 1 shows a schematic of a conventional IBA power supply system 1000, which is an example of a multi-stage power distribution system. More particularly, the power supply system 1000 shown in FIG. 1 is an example of a three-stage power distribution system, wherein power is fed via one or more optional input stage modules to one or more first stage voltage converters, and subsequently to one or more second stage voltage converters. More specifically, in this example, power from mains voltage sources VlineA and VlineB is fed to respective inputs of an input-stage module, which is provided in the exemplary form of a single Power Input Module (PIM) 1100 in the present example. The PIM 1100 may also perform OR-ing between the mains supplies VlineA and VlineB and, in addition, provide filtering and store charge to handle interruptions in power delivery from the mains voltage sources. The power output terminal of the PIM 1100 is connected via a power bus 1200 to the input of at least one first-stage voltage converter, which is provided in the exemplary form of an intermediate Bus Converter (IBC) 1300 in the present example. More generally, a plurality of input-stage modules may be connected via the power bus 1200 to a plurality of first-stage voltage converters. The output of the IBC 1300 is connected via an Intermediate Voltage Bus (IVB) 1400 to a plurality of second-stage voltage converters in the exemplary form of Point-of-Load (POL) regulators 1500, each of which delivers a regulated voltage to one of a plurality of loads 1600. As shown in the example of FIG. 1, any number of POL regulators, 1500-1 to 1500-K, and any number of loads, 1600-1 to 1600-K, may be provided and each load may be connected to a plurality of POL regulators. For simplicity, isolation barriers, Bus drivers, Bus isolators and signal filters are not shown in FIG. 1.
In the interests of maximising the efficiency of the IBC 1300, the IBC 1300 is typically chosen to provide an unregulated output voltage, taking the form of a fixed voltage conversion ratio DC/DC converter. Thus, the IBC 1300 provides a fixed voltage conversion ratio (i.e. input-to-output ratio), most commonly 4:1, 5:1 or 6:1.
In more detail, the IBC 1300 is usually provided in the form of an isolated voltage converter having a primary side circuit, a secondary side circuit and an isolating voltage transformer therebetween. The voltage transformer galvanically isolates the primary side circuit from the secondary side circuit, by AC-coupling the two circuits whilst providing no direct current (DC) path between them. The IBC 1300 thus provides an isolation barrier between the inputs and outputs of the IBA power supply system 1000, with there being no DC connection between the IBC's primary side circuit and secondary side circuit. Providing such an isolation barrier at the IBC 1300 is more cost-effective than providing isolation in each of the POL regulators 1500-1 to 1500-K, owing to the cost of manufacturing isolating voltage transformers.
The primary side circuit of the IBC 1300 is connected to the input of the IBC 1300 and contains at least one switching element which is repeatedly switched with the required switching duty cycle (typically set close to 100%, in order to maximise the IBC's efficiency) to produce an alternating voltage across a primary winding of the voltage transformer. The voltage transformer AC couples the primary and secondary windings, and the secondary side circuit of the IBC 1300 rectifies the voltage induced in the secondary winding of the voltage transformer to produce the output voltage that is output by the IBC 1300 to the IVB 1400. The output voltage of the IBC 1300 varies with the input voltage, Vin, as nxVin, where n is the transformer turns ratio.
Other conventional IBA power supply systems employ a semi-regulated IBC instead of a fixed voltage conversion ratio IBC as described above. In these systems, the semi-regulated IBC provides line regulation to compensate for variations in the IBC's input voltage but at the expense of varying the switching duty cycle, which reduces power efficiency. Furthermore, the IBC's load current affects the IBC's output voltage, with the output voltage decreasing with increasing load current—a phenomenon widely referred to as “droop”. The IBC's output voltage is then given by Vzero—Rdrooplload, where Vzero is the idling voltage at zero load, and Rdroop is the equivalent resistance that consists of different internal parasitic resistances that make the output voltage drop as the load current increases.
As a further alternative, the IBC 1300 may be quasi-regulated, applying line regulation in only a part of the input voltage range, while remaining unregulated with a switching duty cycle close to 100% in other parts of the input voltage range. This control scheme yields an increased input voltage range without increasing the output voltage range.
Examples of conventional IBA power supply systems and IBCs are given in U.S. Pat. Nos. 7,787,261; 7,272,021; 7,558,083; 7,564,702; and 7,269,034.
In general, each of the second stage DC/DC converters may be isolated or non-isolated. However, where isolation is provided by the IBC 1300, the POL regulators 1500-1 to 1500-K are preferably all non-isolated. A second stage DC/DC converter may take the form of an SMPS or alternatively a non-switched linearly-regulated Low Drop Out (LDO) regulator. Each POL (k) delivers a regulated voltage Vload_k to its load 1600-k. In the example of FIG. 1, POL regulators 1500-1 and 1500-2 deliver power to a common load 1600-1 (although, naturally, more than two POL regulators may deliver power to a common load). With the step-down ratio of the IBC 1300 fixed at a pre-selected value, the voltage VIB on the IVB will, of course, vary with changes in the input voltage to the IBC 1300, thus requiring the POL converters to be capable of operating over a range of input voltages, typically 3-15 V.
Although the POL regulators 1500-1 to 1500-K are buck regulators in the example of FIG. 1, their topology is not limited to such and may alternatively be Boost, Buck-Boost etc.
During normal operation of the IBA power supply system 1000, fluctuations in the power bus voltage Vin frequently occur, which may be large enough to damage the IBC 1300. Furthermore, as the IBC 1300 may transmit a damped form of these voltage fluctuations to the IVB 1400 (by virtue of the IBC's fixed voltage transformation ratio), damage may also be sustained by the downstream components of the IBA power supply system, as these components (including the POL regulators) may be forced to operate at voltages that are outside their rated ranges. As the IVB 1400 often as a large decoupling capacitance in conventional IBA power supply systems, the voltage fluctuations on the IVB 1400 may also cause high inrush currents to flow in the IVB 1400, which may damage some of the IBA power supply system components connected to it. In addition, transients on the IVB 1400 that occur on too short a timescale for a POL regulator's feedback loop to respond to may be transmitted to the POL regulator's output, and thus potentially damage the POL regulator's load. Similar problems may also occur during start-up of the IBA power supply system 1000 or during other step changes in the input voltage Vin.
The IBA power supply system 1000 may employ one or more safety mechanisms to protect itself (or its loads) from sustaining damage in one of these ways.
For example, an excessively high current flowing into, or out of, the IBC 1300 may cause damage to the IBC 1300 or the downstream components. In order to reduce the risk of damage, the IBC 1300 typically includes an Over Current Protection (OCP) circuit that shuts down the IBC 1300 when the input current level, or the output current level, exceeds respective threshold levels.
In a similar way, to reduce the risk of damage being caused by excessively high voltages, the POL regulators 1500-1 to 1500-K may each have an Over Voltage Protection (OVP) circuit for shutting down the POL regulator when an excessively high voltage is detected. For example, each of the POL regulators 1500-1 to 1500-K may have an input OVP circuit for shutting down the POL regulator when the voltage on the IVB 1400 is above the maximum operating voltage of the POL regulator. Similarly, to provide protection for the load circuitry, each of the POL regulators 1500-1 to 1500-K may have an output OVP circuit for shutting down the POL regulator when the POL regulator's output voltage is higher than a threshold.
On the other hand, to protect against damage due to an excessively low IVB voltage, each of the POL regulators 1500-1 to 1500-K may have an Under Voltage Lock-Out protection (UVLO) circuit, which shuts down the POL regulator when the IVB 1400 voltage is below the minimum operating voltage of the regulator 1500.
Although protection circuits of these kinds can provide effective protection for the IBA power supply system 1000, they have the disadvantage of causing the IBA power supply system 1000 (or some components thereof) to shut down in response to voltage changes or fluctuations that might otherwise be safely tolerated. For example, a large inrush current which can occur during a voltage transient or at start-up of the IBA power supply system 1000 may activate the OCP circuit in the IBC 1300, causing the IBC 1300, and thus the IBA power system 1000 as a whole in the present example, to shut down. Voltage transients on the IVB 1400 may trigger operation of the POL regulators' input OVP or UVLO circuits, causing the affected POL regulators to shut down, potentially initiating a shut-down or a restart of the IBA power supply system 1000 as a whole.
Regardless of whether they utilize protection circuits of these kinds, conventional IBA power supply systems also suffer from problems with electromagnetic interference (EMI) caused by transients on the power bus 1200 or transients in the load current of one or more of the POL regulators 1500-1 to 1500-K, which lead to oscillations on the IVB 1400 that are damped only by the inherent parasitic resistances in the IBC's output filter circuit. These problems increase when the IBA power supply system 1000 comprises a high efficiency IBC with a low parasitic resistance in its output choke, and also with increasing use of decoupling banks comprising ceramic capacitors with low equivalent series resistance (ESR).
Thus, many conventional IBA power supply systems are susceptible to high levels of EMI, and are not robust to transients or abrupt changes in the systems' input voltage or load level. As a consequence, many conventional IBA power supply systems are prone to sustaining damage or, where one or more of the above-mentioned protection circuits are employed, shutting down or restarting unnecessarily.